[Juicy-Sweet]

Originally Posted by Puddleglum

He doesn't even present it as an estimate, simply a statement. Nothing I am aware of, either in Tolkien's writing or in our knowledge of midieval history (the closest in level of advancement to Middle Earth thru the 3rd age) would seem to support such a population, or anything even on that order of magnitude.

Even 10,000, for a cave-dwelling population, seems high - but even if you go with that population, there seems no indication the Balrog initiated a direct frontal assault. He lives in a realm of tunnels which he knows very well (especially the lower ones). He also has some level of (what might be thought of as magical) arts - witness him perceiving Gandalf's "shutting spell" and delivering a counter-spell that breaks Gandalf's control.

So, I'd imagine him sneaking around, finding and killing Dwarves where he finds them, and, when approached by a force, slashing them, bashing them, and disappearing back into the bowels of the earth. In a tunnel, the Dwarves can't surround him and attempt to overwhelm him with superior numbers.

Essentially, each confrontation becomes something like 20 to 1 (or even 50 to 1). If the Balrog can win that fight, and maintain his stamina, he can kill Thousands of Dwarves - 20 at a time.

Until, finally, the Dwarves realize they can't keep that up.

London and Paris were at 200 000 in the middle ages at some point.

Killing 20 dwarfs a day on an average day, he would get 7300 in a year. Funnily that adds up pretty well with the 10 000 population.